Soap vs. oil vs. water, and polar-non-polar review:
Water alone is not able to penetrate grease or oil because they are of
opposite polarity.
When grease or oil (non-polar hydrocarbons) are mixed with a soap- water
solution, the soap molecules work as a "bridge" between polar water
molecules and non-polar oil molecules. Soap molecules have both properties
of non-polar and polar at opposite ends of the molecule.
The oil is a pure hydrocarbon so it is non-polar. The non-polar hydrocarbon
tail of the soap dissolves into the oil. That leaves the polar carboxylate ion of
the soap molecules are sticking out of the oil droplets, the surface of each oil
droplet is negatively charged. As a result, the oil droplets repel each other
and remain suspended in solution (this is called an emulsion) to be washed
away by a stream of water. The outside of the droplet is also coated with a
layer of water molecules.
The graphics below, although not strictly a representation of the above
description is a micelle that works in much the same fashion. The oil would
be a the center of the micelle(A micelle is a group of fatty acids, or lipids
which clump together to the exclusion of water).
How do amphiphilic molecules arrange themselves in water? We have seen
that in oil and water mixtures they form micelles and monolayers which
keep the hydrophilic heads in water and the hydrophobic tails away from the
water. Without any oil this means that the tails will tend to go next to each
other to avoid the water.
We have seen the micelle before but the molecules can also for a double
layer structure:
This arrangement is called a bilayer. Imagine two sheets of molecules
sandwiched together so that all the hydrophilic heads on the outside (in
contact with the water) and the hydrophobic tails inside (away from the
water).
Just like monolayers, we say that bilayers are membranes. Bilayers are
particularly interesting because of their properties and their importance in
biological systems. The human body depends on many many membranes of
this type.
So, back when you were in grade school, did the girls mix with the boys in
general? Probably not.
Well, at your school, do the boys and girls mix? Probably not.That is not
because the girls don't like the boys (usually), or because the boys don't like
the girls. It's just that young girls like spending time with other young girls
more than with young boys. Boys and girls tend to like to do different things
and talk about different things when they are young. Imagine you are a girl
going outside for recess. You look around. You see a boy or two, and a girl
or two. Naturally you go over to the girl to talk. Later, another girl comes
out. She, too, goes over to talk to the girls. After a while, after every child is
outside, you have patches of boys and patches of girls all over the place, but
not many areas where boys and girls are together.
The same thing happens with oil and water. Oil is made of molecules (which
are like very small, sticky, rubber balls). So is water. But water molecules
are not the same as oil molecules. Most importantly, while all molecules like
to stick to each other, oil molecules like to stick to other oil molecules more
than they like to stick to water molecules. Same with the water: water
molecules like to stick to oil molecules, but they like to stick to other water
molecules even more. So, if you pour some water into some oil, the water
molecules coming in see oil molecules and (at first only a few) water
molecules. Naturally they prefer to join the other water molecules. After all
the water is in, you have patches of water, and patches of oil, but nowhere
the two kinds of molecules mixed up together. It turns out that oil is lighter
than water, so that the patches of oil tend to float up on top of the water, and
join together into a big oil slick. If you get enough oil, you get a thick layer
of it on top of the water.
You can sort of mix up the oil and water by shaking the jar, but you will
only break up the layer into smaller patches, and they will come back
together again quickly because of the lightness business. Now, if you were
on the Space Shuttle, the patches wouldn't form a thick layer, they'd just stay
all jumbled up. But the oil and water wouldn't mix! You CAN force oil and
water to mix. What you need is another type of molecules which both the
water and the oil like to stick to more than they like to stick to each other.
Such a molecule is called an emulsifier, or, more simply, a soap. When you
get a soap molecule in there, the water and the oil stick to it, and then the oil
and water mix. A lot of things can be emulsifiers. Soap is one of them. If
you get grease on your hands, plain water won't wash it off -- the grease
won't mix with the water and come off. But if you put soap on your hands,
the grease will mix with the water, and come off. Egg yolks are another
thing that works. So if you mix salad oil and vinegar (which is mostly
water), and then put some eggs in, the oil and vinegar will mix -- you get
mayonnaise.
Water is a polar molecule which means that it has a negatively charged end
and a positively charged end. So water molecules attract each other. They
also attract other polar molecules. Oil is NOT a polar molecule-it doesn't
have a separation of charge. So water and oil aren't attracted to each other.
Just remember-like dissolves like.
Effect of Hard Water:
If soap is used in "hard" water, the soap will be precipitated as "bath-tub
ring" by calcium or magnesium ions present in "hard" water.
The effects of "hard" water calcium or magnesium ions are minimized by
the addition of "builders". The most common "builder" used to be sodium
trimetaphosphate. The phosphates react with the calcium or magnesium ions
and keeps them in solution but away from the soap molecule. The soap
molecule can then do its job without interference from calcium or
magnesium ions. Other "builders" include sodium carbonate, borax, and
sodium silicate are currently in detergents.
What is a surfactant?
A surfactant or surface active agent is a substance that, when dissolved in
water, gives a product the ability to remove dirt from surfaces such as the
human skin, textiles, and other solids.
In more technical terms:
they enable the cleaning solution to fully wet the surface being cleaned so
that dirt can be readily loosened and removed.
they clean greasy, oily, particulate-, protein-, and carbohydrate-based
stains.
they are instrumental in removing dirt and in keeping them emulsified,
suspended, and dispersed so they don't settle back onto the surface being
cleaned.
Each surfactant molecule has a hydrophilic (water-loving) head that is
attracted to water molecules AND a hydrophobic (water-hating) tail that
repels water and simultaneously attaches itself to oil and grease in dirt.
These opposing forces loosen the dirt and suspend it in the water. The
mechanical agitation of the washing machine helps pull the dirt free.
Surfactants are one of the major components of cleaning products and can be
regarded as the 'workhorses': they do the basic work of breaking up stains
and keeping the dirt in the water solution to prevent re-deposition of the dirt
onto the surface from which it has just been removed. Surfactants disperse
dirt that normally does not dissolve in water.
As anyone who uses oil based dressings in the kitchen knows, oil and water
do not mix unless shaken vigorously in the bottle. They separate almost
immediately afterwards. The same is true when washing your dishes or
clothes. With the addition of surfactants, oil, which normally does not
dissolve in water, becomes dispersible and can be removed with the wash
water.
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What does a surfactant actually do?
Surfactants are also referred to as wetting agents and foamers. Surfactants
lower the surface tension of the medium in which it is dissolved. By
lowering this interfacial tension between two media or interfaces (e.g.
air/water, water/stain, stain/fabric) the surfactant plays a key role in the
removal and suspension of dirt. The lower surface tension of the water
makes it easier to lift dirt and grease off of dirty dishes, clothes and other
surfaces, and help to keep them suspended in the dirty water. The waterloving or hydrophilic head remains in the water and it pulls the stains
towards the water, away from the fabric. The surfactant molecules surround
the stain particles, break them up and force them away from the surface of
the fabric. They then suspend the stain particles in the wash water to remove
them.
More in-depth understanding of Surface tension:
http://www.ilpi.com/genchem/demo/tension/.
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What does a surfactant "look like"?
A tadpole! A surfactant consists of a hydrophobic (non-polar) hydrocarbon
"tail" and a hydrophilic (polar) "head" group.
This appearance is key to its behaviour. The dirt-loving or hydrophobic tail
absorbs to the oil and grease in dirt and stains.
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Are surfactants of natural or synthetic origin ?
They can be either. Surfactants from natural origin (vegetable or animal) are
known as oleo-chemicals and are derived from sources such as palm oil or
tallow. Surfactants from synthetic origin are known as petro-chemicals and
are derived from petroleum.
Oleo-Chemicals
Petro-Chemicals
Having the flexibility to use both oleochemical and petrochemical
surfactants allows our formulators to create products that maximize the
value in the bottle of detergent, so to speak, by optimizing cleaning ability
under a variety of laundry conditions while keeping the price low in the
current market. These days, our formulation scientists focus quite a lot on
developing detergents that perform well at lower wash temperatures. This
approach will continue to yield energy savings during the consumer use
phase, hence a reduction of CO2 emissions.
Surfactants also have an important role in our body, where they are used to
reduce surface tension in the lungs. The human body does not start to
produce lung surfactants until late in foetal development. Therefore,
premature babies are often unable to breathe properly, a condition called
Respiratory Distress Syndrome. Untreated, this is a serious illness and is
often fatal, but administration of artificial surfactants virtually eliminates
this health problem.
Want to read more on oleo - versus petro-chemicals? continue.
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Are there different types of surfactants?
There is a broad range of different surfactant types, each with unique
properties and characteristics: the type of dirt and fabric on which they work
best, how they can cope with water hardness. Detergents use a combination
of various surfactants to provide the best possible cleaning results. There are
four main types of surfactants used in laundry and cleaning products.
Depending on the type of the charge of the head, a surfactant belongs to the
anionic, cationic, non-ionic or amphoteric/zwitterionic family.
Anionic surfactants
In solution, the head is negatively charged. This is the most widely used type
of surfactant for laundering, dishwashing liquids and shampoos because of
its excellent cleaning properties and high . The surfactant is particularly
good at keeping the dirt away from fabrics, and removing residues of fabric
softener from fabrics.
Anionic surfactants are particularly effective at oily soil cleaning and
oil/clay soil suspension. Still, they can react in the wash water with the
positively charged water hardness ions (calcium and magnesium) , which
can lead to partial deactivation. The more calcium and magnesium
molecules in the water, the more the anionic surfactant system suffers from
deactivation. To prevent this, the anionic surfactants need help from other
ingredients such as builders (Ca/Mg sequestrants) and more detergent should
be dosed in hard water.
The most commonly used anionic surfactants are alkyl sulphates, alkyl
ethoxylate sulphates and soaps.
Cationic surfactants
In solution, the head is positively charged. There are 3 different categories of
cationics each with their specific application:
In fabric softeners and in detergents with built-in fabric softener, cationic
surfactants provide softness. Their main use in laundry products is in rinse
added fabric softeners, such as esterquats, one of the most widely used
cationic surfactants in rinse added fabric softeners.
An example of cationic surfactants is the esterquat.
In laundry detergents, cationic surfactants (positive charge) improve the
packing of anionic surfactant molecules (negative charge) at the stain/water
interface. This helps to reduce the dirtl/water interfacial tension in a very
efficient way, leading to a more robust dirt removal system. They are
especially efficient at removing greasy stains.
An example of a cationic surfactant used in this category is the mono alkyl
quaternary system.
In household and bathroom cleaners, cationic surfactants contribute to the
disinfecting/sanitizing properties.
Non-ionic surfactants
These surfactants do not have an electrical charge, which makes them
resistant to water hardness deactivation. They are excellent grease
removers that are used in laundry products, household cleaners and hand
dishwashing liquids.
Most laundry detergents contain both non-ionic and anionic surfactants as
they complement each other's cleaning action. Non-ionic surfactants
contribute to making the surfactant system less hardness sensitive.
The most commonly used non-ionic surfactants are ethers of fatty alcohols
Amphoteric/zwitterionic surfactants
These surfactants are very mild, making them particularly suited for use in
personal care and household cleaning products. They can be anionic
(negatively charged), cationic (positively charged) or non-ionic (no charge)
in solution, depending on the acidity or pH of the water.
They are compatible with all other classes of surfactants and are soluble
and effective in the presence of high concentrations of electrolytes, acids
and alkalis.
These surfactants may contain two charged groups of different sign.
Whereas the positive charge is almost always ammonium, the source of the
negative charge may vary (carboxylate, sulphate, sulphonate). These
surfactants have excellent dermatological properties. They are frequently
used in shampoos and other cosmetic products, and also in hand
dishwashing liquids because of their high foaming properties.
An example of an amphoteric/zwitterionic surfactant is alkyl betaine.
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How do surfactants work in detail?
Surfactants can work in three different ways: roll-up, emulsification, and
solubilization.
Roll-up mechanism
The surfactant lowers the oil/solution and fabric/solution interfacial tensions
and in this way lifts the stain of the fabric.
Emulsification
The surfactant lowers the oil-solution interfacial tension and makes easy
emulsification of the oily soils possible.
Solubilization
Through interaction with the micelles of a surfactant in a solvent (water), a
substance spontaneously dissolves to form a stable and clear solution.
Read more on how surfactants form suds.
http://www.exploratorium.edu/ronh/bubbles/bubbles.html
http://science.nasa.gov/newhome/headlines/AST05MAR98_3.htm
Read more about what water hardness is and how it affects cleaning.
http://www.systemsaver.com/windsor-website/education/education.html
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How can surfactants prevent dirt from being re-deposited?
Surfactants have a vital role to play in preventing the re-deposition of soils
like greasy, oily stains and particulate dirt on the surface or fabric from
which they have just been removed. This works by electrostatic interactions
and steric hindrance.
Electrostatic interactions
Anionic surfactants are adsorbed on both the surface of the dirt which is
dispersed in the detergent solution, and the fabric surface. This creates a
negative charge on both surfaces, causing electrostatic repulsions. This
repulsion prevents the soil from re-depositing on the fabric.
In the presence of hardness, however, this mechanism acts like a 'bridge'
between the suspended soil and the fabric. This is another reason why
hardness sequestrants (a chemical that promotes Ca/Mg sequestration) are
often used in detergents.
Steric hindrance:
Non-ionic surfactants like alcohol ethoxylates also adsorb on the dirt. Their
long ethoxylated chains extend in the water phase and prevent the dirt
droplets or particles from uniting,, and from depositing onto the fabric
surface. This is shown in the illustration below: (1) Dirt is present in solution
(2) The non-ionic surfactants adsorb to the dirt particles. (3) Their long
hydrophilic heads extend in the water phase and as a result prevent the dirt
particles/droplets from uniting and from re-depositing onto fabrics.
Soaps are sodium or potassium fatty acids salts, produced from the
hydrolysis of fats in a chemical reaction called saponification. Each soap
molecule has a long hydrocarbon chain, sometimes called its 'tail', with a
carboxylate 'head'. In water, the sodium or potassium ions float free, leaving
a negatively-charged head.
Soap is an excellent cleanser because of its ability to act as an emulsifying
agent. An emulsifier is capable of dispersing one liquid into another
immiscible liquid. This means that while oil (which attracts dirt) doesn't
naturally mix with water, soap can suspend oil/dirt in such a way that it can
be removed.
The organic part of a natural soap is a negatively-charged, polar molecule.
Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water
molecules via ion-dipole interactions and hydrogen bonding. The
hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar
hydrocarbon chain, does not interact with water molecules. The hydrocarbon
chains are attracted to each other by dispersion forces and cluster together,
forming structures called micelles. In these micelles, the carboxylate groups
form a negatively-charged spherical surface, with the hydrocarbon chains
inside the sphere. Because they are negatively charged, soap micelles repel
each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap and soiling
oils are mixed, the nonpolar hydrocarbon portion of the micelles break up
the nonpolar oil molecules. A different type of micelle then forms, with
nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt'
attached to them are caught inside the micelle and can be rinsed away.
Although soaps are excellent cleansers, they do have disadvantages. As salts
of weak acids, they are converted by mineral acids into free fatty acids:
CH3(CH2)16CO2-Na+ + HCl ----> CH3(CH2)16CO2H + Na+ + ClThese fatty acids are less soluble than the sodium or potassium salts and
form a precipitate or soap scum. Because of this, soaps are ineffective in
acidic water. Also, soaps form insoluble salts in hard water, such as water
containing magnesium, calcium, or iron.
2 CH3(CH2)16CO2-Na+ + Mg2+ ----> [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce hair luster,
and gray/roughen textiles after repeated washings. Synthetic detergents,
however, may be soluble in both acidic and alkaline solutions and don't form
insoluble precipitates in hard water. But that is a different story...